A well-known hazard associated with exposure to the space environment is the risk of vehicle failure due to an impact from a micrometeoroid and orbital debris (MMOD) particle. Among the vehicles of importance to NASA is the extravehicular mobility unit (EMU) spacesuit used while performing a US extravehicular activity (EVA). An EMU impact is of great concern as a large leak could prevent an astronaut from safely reaching the airlock in time resulting in a loss of life. For this reason, a risk assessment is provided to the EVA office at the Johnson Space Center (JSC) prior to certification of readiness for each US EVA

Christiansen, Eric L.

Hoffman, Kevin D.

Hyde, James L.

Lear, Dana M.

NASA Technical Reports Server

Proceedings of the 2019 Hypervelocity Impact Symposium 
HVIS2019 
April 14-19, 2019, Destin, FL, USA 
HVIS2019-XXX 
  
Extravehicular Activity Micrometeoroid and Orbital Debris Risk 
Assessment Methodology 
Kevin D. Hoffmana, James L. Hydea, Eric L. Christiansenb, and Dana M. Learb,  
aJacobs, NASA Johnson Space Center, Mail Code XI, Houston, TX  77058, USA 
bNASA Johnson Space Center, Mail Code XI, Houston, TX  77058, USA  
Abstract 
A well-known hazard associated with exposure to the space environment is the risk of vehicle failure due to an impact from a micrometeoroid 
and orbital debris (MMOD) particle.  Among the vehicles of importance to NASA is the extravehicular mobility unit (EMU) “spacesuit” used 
while performing a US extravehicular activity (EVA). An EMU impact is of great concern as a large leak could prevent an astronaut from safely 
reaching the airlock in time resulting in a loss of life.  For this reason, a risk assessment is provided to the EVA office at the Johnson Space 
Center (JSC) prior to certification of readiness for each US EVA. 
Keywords: Extravehicular activity; Micormeteoroid; Orbital debris  
1. EVA Assessment Overview 
EVA assessments are performed using standard risk assessment methodology illustrated in Figure 1 (Christiansen, 2003).  
Central to the process is the Bumper risk assessment code (Figure 2), which calculates the critical penetration risk based on 
geometry, shielding configurations and flight parameters.  
 
The assessment process begins by building a Finite Element Model (FEM) of the EMU, which defines the size and shape of 
the EMU as well as the locations of the various shielding configurations. Each region of the EMU having a different shield 
configuration and/or failure criteria are given a unique property identifier (PID) number in the FEM. The model was built using 
the NX I-deas software package from Siemens PLM Software. The FEM was constructed using triangular and quadrilateral 
elements that define the outer shell of the EMU. Bumper-3 uses the model file to determine the geometry of the EMU for the 
analysis.  
 
The next step of the process is to identify the failure criteria and appropriate ballistic limit characteristics for the various regions 
of the FEM. The two failure criteria for the EMU are perforation threshold failure and a critical hole size threshold failure of the 
EMU bladder.  Perforation threshold failure would result in any size leak in the EMU bladder while the critical hole size threshold 
failure (>4mm hole in the bladder) would result in an uncontrolled leak. The ballistic limit equations define the critical size particle 
that will cause failure (as defined by the failure criteria) at a given impact angle and impact velocity. When the finite element 
model is built, each individual element is assigned a PID to act as an index for its shielding properties.  Ballistic limit equations 
are based on results from impact tests on samples that are representative of the vehicle shields. The appropriate ballistic limit 
equations (BLEs) are added to the Bumper-3 code as needed for each and every PID in the model.  
 
The final stage of the analysis is to determine the predicted number of failures for the EMU. This is calculated using the 
micrometeoroid and orbital debris (MMOD) environment definitions that are specified by the spacecraft design. The MMOD 
environment models predict an impacting particle flux based on inputs for orbital parameters, spacecraft attitude and analysis date. 
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K.D. Hoffman, J.L. Hyde, et.al. / HVIS Proceedings Vol 15 (2019) 000–000 
Using Poisson statistics, and combining the previously defined geometry, shielding characteristics and ballistic limit equations 
with the MMOD environment model calculations, the Bumper-3 code determines a predicted number of failures for the EMU. 
 
     
Fig. 1. MMOD Risk Assessment Flowchart 
     
Fig. 2. Bumper-3 MMOD Risk Assessment Code 
1.1. US EVA Timeline Analysis 
For every US EVA, a timeline MMOD risk analysis is provided to the EVA office at JSC prior to the certification of EVA 
readiness. With assistance from the EVA office, a summary of EMU positions (including body orientation) for the specific EVA 
worksite locations on the International Space Station (ISS) is produced using detailed EVA summaries/presentations and/or EVA 
training run videos from the Neutral Buoyancy Lab (NBL.) From this information, the analysis FEMs are built by orienting an 
K.D. Hoffman, J.L. Hyde, et.al. / HVIS Proceedings Vol 15 (2019) 000–000 
EMU FEM at each worksite location on a simplified ISS FEM. When one or more worksites require multiple body orientations, 
additional analysis FEMs are built. The analysis FEM used for the US 52 and 53 EVAs is shown in Figure 3.       
   
Fig. 3. Increment 57 (US EVAs 52 & 53) Analysis Model including ISS Shadowing 
The analysis FEM(s), environment files and BLE parameters are inputted into the Bumper-3 code to obtain the expected number 
of failures at each worksite for a one year exposure. These results are scaled based on actual worksite exposure duration to generate 
total EVA risk and individual EMU risk for two failure criteria (any size leak and catastrophic leak.) Meteoroid and orbital debris 
enhancement factors (see Section 1.2) are applied to obtain the failure odds for the planned EVA date.            
1.2. Meteoroid and Orbital Debris Enhancement Factors 
The meteoroid and orbital debris environment files used for the timeline analysis provide the average flux for a single year 
exposure duration. Due to the short exposure duration of 6 to 7 hours, EVAs are more sensitive to sporadic events outside of the 
single year background environment (i.e. orbital debris breakups and meteor showers.) Environment enhancement factors are used 
to account for these events. For every US EVA, the Meteoroid Environment Office (MEO) at Marshall Space Flight Center 
(MSFC) provide 6-hour meteoroid enhancement factors for each day of the EVA window. The fluence factor is for particles with 
a kinetic energy of at least 6.7 J (the kinetic energy of a 0.04 cm diameter meteoroid traveling at 20 km/s.) For example, meteoroid 
enhancement factors provided by the MEO for the December 2018 window are shown in figure 4 and indicate a peak fluence 
enhancement factor of nearly 40% for an EVA performed during the Geminids meteor shower on December 14th, 2018. Likewise, 
the Orbital Debris Program Office (ODPO) at JSC provide an orbital debris enhancement factor to account for any recent orbital 
debris breakups that might increase the flux at ISS orbit. 
K.D. Hoffman, J.L. Hyde, et.al. / HVIS Proceedings Vol 15 (2019) 000–000 
   
Fig. 4. Increment 57 (US EVAs 52 & 53) Meteoroid Environment Enhancement Factors 
2. EMU Finite Element Model 
A detailed finite element model (FEM) of the EMU (shown in Figure 5) has been created which has regions for the various 
shielding configurations. Each shielding configuration is based on the layers and materials over the innermost bladder layer that 
maintains the acceptable atmospheric environment for the astronaut. The EMU outer mold line is defined by over 14,500 
quadrilateral and triangular elements. The color changes in the model views shown in Figure 5 illustrate the 42 different surface 
property ID (PID) types. 
   
Fig. 5. Extravehicular Mobility Unit (EMU) Finite Element Model (FEM) 
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3. Hypervelocity Impact Testing 
The EMU can be subdivided into two major shielding configurations: soft goods and hard goods. The soft goods (fabric layers 
over inner bladder) include the arms, legs, lower torso, gloves and boots overlap regions while the hard goods include the helmet, 
face plate, sizing rings, hard upper torso (HUT), portable life-support system (PLSS), and the display and control module (DCM.)  
65 “soft goods” and over 40 “hard goods” hypervelocity impact tests were performed on EMU samples at the White Sands Test 
Facility. Testing of “soft goods” included both pressurized and unpressurized bladders with no difference found in the ballistic 
limit with pressurization. Testing results were used to set the ballistic limit equations (BLE) for each shielding configuration.  
4. EMU Random Orientation Risk Study 
A risk assessment of the EMU was performed using the ORDEM 3.0 orbital debris environment and the MEM R2 meteoroid 
environment. Twenty-four distinct attitude (body orientation) runs of the unshadowed EMU FEM were made for the exposure 
period of 2018 to 2024. Figure 6 shows the percentage of total risk contribution for the main regions of the EMU, with 75% of 
the total risk from the soft goods regions of the lower torso assembly, arms and gloves. The risk to area ratio for each of these 
sections is shown in Figure 7 and highlights that the gloves carry the highest risk per area for the EMU. Figure 8 is a risk contour 
(red areas are higher risk, blue areas are lower risk) and indicates the soft good regions of the gloves, arms, legs and torso carry 
the most risk.   
   
Fig. 6. Extravehicular Mobility Unit (EMU) Unshadowed Penetration Risk Summary (ORDEM3 and MEM R2 environments) 
K.D. Hoffman, J.L. Hyde, et.al. / HVIS Proceedings Vol 15 (2019) 000–000 
   
Fig. 7. Extravehicular Mobility Unit (EMU) Unshadowed Penetration Risk to Region Area Summary (ORDEM3 and MEM R2 environments) 
 
   
Fig. 8. Extravehicular Mobility Unit (EMU) Unshadowed Average Risk Contour Plot (red areas = higher risk, blue areas = lower risk) 
K.D. Hoffman, J.L. Hyde, et.al. / HVIS Proceedings Vol 15 (2019) 000–000 
Acknowledgements 
In support of the Jacobs JSC Engineering, Technology and Science (JETS) contract (NNJ13HA01C) for the Astromaterials 
Research and Exploration Science (ARES) division at the NASA Johnson Space Center (JSC.) 
References 
[1] Christiansen, Eric L., 2003. Meteoroid/Debris Shielding, NASA TP-2003-210788 . Houston: NASA. 
[2] Friesen, L.J. and Christiansen, E.L., 1997. Hypervelocity Impact Tests of Extravehicular Mobility Unit (EMU) (Space Suit) Material Samples, Part 1,  
JSC 27856, Houston: NASA 
[3] Lyons, Frankel, 1998. Hypervelocity Impact Testing of Extravehicular Mobility Unit (EMU) Alternative Material Samples, Part 1, JSC 28445, Houston: NASA 
 


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